The present invention relates to a preform, made of short fibers, for incorporation into a composite material for providing fiber reinforcement therefor; to said fiber reinforced composite material incorporating said short fiber preform, itself; and to a method for making said short fiber preform.
In order to improve the strength of materials, particularly of light metals such as aluminum or aluminum alloy, and in order to improve their heat resistance, their wear resistance, and so on, so as to allow various components made from the materials to be made lighter, it has been considered in various fields to provide fiber reinforcement such as alumina fibers as embedded in the materials. Thus, composite reinforced materials have been proposed, and utilized, composed of a matrix of metal which has been a light metal such as aluminum alloy, with embedded in it reinforcing fibers such as alumina fibers. And, particularly, short fibers have been used as such reinforcing fibers. Further, a conventional method for manufacturing such a composite reinforced material has been the pressurized casting method, in which first by the so called compression forming method or by the so called suction method a preform has been made as composed of many such short fibers such as alumina fibers stuck together by a binder, and then the matrix metal such as aluminum alloy in molten form has been poured around the preform and has been caused to pemeate into its interstices under pressure.
However, with regard to a part made of such fiber reinforced composite material, there has been a basic conflict in the properties required from such a part, which has entailed compromises to be made during design. Namely, the heat resistance and the strength and the wear resistance of such a fiber reinforced composite material part are generally the greater, the greater is the amount of reinforcing fiber material incorporated therein, in other words the greater is the volume proportion of the reinforcing fibers in the preform from which the composite material is produced by the pressurized casting method as detailed above; but, on the other hand, the greater is said volume proportion of the reinforcing fibers in the preform, the more difficult does the fiber reinforced composite material become to machine--i.e. its workability decreases--and the greater becomes the amount of wear on a mating member which rubbingly slides against and cooperates with a part made from said fiber reinforced composite material. In other words, a part made from a short fiber preform which has a relatively high fiber volume proportion is relatively strong, and has relatively good heat resistance and relatively good wear resistance; but, as disadvantages, its machinability and workability are relatively poor and the rate of wear on a tool which is used for working it is relatively high, and further the amount of wear on a mating member which slidingly cooperates with said part is relatively high. On the other hand, a part made from a short fiber preform which has a relatively low fiber volume proportion is relatively weak, and has relatively poor heat resistance and relatively poor wear resistance; but, as advantages, its machinability and workability are relatively good and the rate of wear on a tool which is used for working it is relatively low, and further the amount of wear on a mating member which slidingly cooperates with said part is relatively low.
With the object of reconciling these conflicting requirements for characteristics of a part made from a short fiber reinforced material, it has been observed that typically different requirements are made of the different portions of such a part, and accordingly a solution which has been practiced has been to make the short fiber preform from which the part is made in different portions which are of different volume proportions. Thereby, one portion of the short fiber preform is made to be of a high volume proportion, and this portion of the short fiber preform, when the matrix metal has been infiltrated thereinto and when the finished part has been machined, constitutes a portion of said finished part which is demanded to be of relatively high strength and relatively high heat resistance and relatively high wear resistance, but which is not required to have particularly good characteristics with regard to wear on a mating or cooperating member, or machinability or workability; while, on the other hand, another portion of the short fiber preform is made to be of a low volume proportion, and this portion of the short fiber preform, when the matrix metal has been infiltrated thereinto and when the finished part has been machined, constitutes a portion of said finished part which is not particularly demanded to be of relatively high strength or relatively high heat resistance or relatively high wear resistance, but which is required to have good characteristics with regard to wear on a mating or cooperating member, and which is required to have good machinability and workability.
However, in the prior art this solution has been of limited applicability. It has been practiced to form the short fiber preform in two separate parts of radically different fiber volume proportions, in order as outlined above to obtain acceptable characteristics from the different portions of a finished part made from composite material incorporating the preform, but the problem arises that, when a first short fiber preform portion (typically made by the compression forming method) having relatively high volume proportion is assembled together with a second short fiber preform portion (typically made by the suction method) having relatively low volume proportion, and when a composite material is made from these two conjoined preform portions and a part is made from the composite material, there is a sudden discontinuous change in the fiber volume proportion at the boundary between the first and the second preform portions, and further there are no reinforcing fibers spanning between these portions, in other words there is inevitably at least a small gap between these portions made substantially only of matrix metal. Further, there is a sudden discontinuous change in the thermal properties of the composite material, such as the thermal expansion coefficient and the thermal conductivity, at the aforesaid boundary. This means that, if the finished part is subjected to severe stresses, and in particular if said finished part is subjected to repeated cyclic application of load or is subjected to a repeated hot and cold cycle, there is a strong likelihood of fracturing occurring between the two portions of said finished part at the boundary defined between the two preform portions. This problem in fact also tends to occur with a part made out of a material which is composite reinforced only in one local portion thereof. Further, such a method for making a composite material out of a pair of conjoined preform portions is by its nature inefficient.